Is javascript "truly parallel"?

Question

Can 2 asynchronous functions execute at the same time? For example in the code below, is it possible that the command let xEquals2 = x === 2; from the first setTimeout gets executed, then the same command from the second setTimeout, and finally the if block from the first setTimeout. Or to make the question simpler will the code below always print out the number 2 two times, or is it possible that it will print out 2 and 3 or 3 and 3?

let x = 1;

setTimeout(() => {
    let xEquals2 = x === 2;
    if (!xEquals2) {
        x++;
    }
    console.log(x);
}, 1000);

setTimeout(() => {
    let xEquals2 = x === 2;
    if (!xEquals2) {
        x++;
    }
    console.log(x);
}, 1000);

You can read about the event loop here. developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop — Cody Geisler
– Cody Geisler, Commented Jan 23, 2018 at 19:27

Daniel A. White · Accepted Answer · 2018-01-23 19:30:46Z

4

JavaScript uses a event loop which is not like threads that other platforms have. Hence the first callback will execute first, then the second, so your result will be 2, 2.

edited Jan 23, 2018 at 19:30

answered Jan 23, 2018 at 19:22

Daniel A. White

192k49 gold badges389 silver badges473 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

displayName Over a year ago

Does that mean once one async function starts executing, it won't get interrupted by any other async function until it's done?

Daniel A. White Over a year ago

@TheOneWhoMade that's not true for javascript.

displayName Over a year ago

@DanielA.White I guess you meant result will be 2, 2? I tried suggesting edit, but it has to be at least 6 characters :)

Daniel A. White Over a year ago

@marko yea, i just had hard time finding your logic.

Aluan Haddad Over a year ago

Note that JavaScript is single threaded. It is important not to confuse concurrency and asynchrony. Interleaved IO provides IO parallelism because the CPU is not waiting for each IO operation to complete. For CPU bound work, JavaScript will limit you to 1 core.

Collectives™ on Stack Overflow

Is javascript "truly parallel"?

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related